Electronic filters may receive an input signal having various frequency components and transmit a corresponding output signal having some or all of the input signal's frequency components. Depending on the type of electronic filter, frequencies within certain designated frequency ranges may be passed while other frequencies are attenuated. Pass band refers to the range of frequencies which may be transmitted and attenuation band refers to the range of frequencies that will be suppressed. Filters may be classified according to the ranges of their pass band and attenuation band. Filters may be generally classified as low-pass, high-pass, band-pass, and band-stop. A low-pass filter passes relatively low frequencies, and a high-pass filter passes relatively high frequencies. A band-pass filter passes the frequencies of a signal within a particular frequency range while attenuating the frequencies of the signal outside of this frequency range. A band-stop filter passes all frequencies of a signal except those frequencies within a particular frequency range.
Applications using electronic filters, such as programmable, electronic filters, often require accurate cut-off frequencies. Cut-off frequencies, when referring to electronic filters, refer to those frequencies that separate the pass band and the attenuation band of the frequency response characteristic of a filter. The desired cut-off frequency may be provided or input into a programmable, electronic filter so the desired filtering effects may be achieved. It is often difficult to accurately establish the desired cut-off frequencies in programmable, electronic filters. Programmable, continuous-time electronic filters often present substantial cut-off frequency difficulties. The difficulties typically arise due to such things as parasitics, capacitor or transistor mismatches, device size differences, and the resolution of programming steps.
The capacitance of capacitors used in programmable, electronic filters may vary as much as plus or minus 20%. This presents problems in accurately establishing cut-off frequencies over a given frequency range. In the tuning loop portion of a programmable, electronic filter, tuning control adjustment may be added to adjust for the variation in capacitance to ensure that one, predefined cut-off frequency may be accurately established. However, the tuning control adjustment provides an accurate cut-off frequency for this one predefined cut-off frequency to which it was originally adjusted and may not necessarily provide an accurate cut-off frequency for other desired cut-off frequency values. Therefore, these types of programmable, electronic filters do not work well in applications requiring a range of accurate cut-off frequencies. Establishing accurate cut-off frequencies becomes especially difficult when small cut-off frequency changes are required or when non-standard cut-off frequencies are desired. These difficulties may also arise due to resolution or granularity problems.